Inspection apparatus and inspection method

ABSTRACT

A method of inspection for defects on a substrate, such as a reflective reticle substrate, and associated apparatuses. The method includes performing the inspection using inspection radiation obtained from a high harmonic generation source and having one or more wavelengths within a wavelength range of between 20 nm and 150 nm. Also, a method including performing a coarse inspection using first inspection radiation having one or more first wavelengths within a first wavelength range; and performing a fine inspection using second inspection radiation having one or more second wavelengths within a second wavelength range, the second wavelength range comprising wavelengths shorter than the first wavelength range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of EP application 18154116.0, which wasfiled on 30 Jan. 2018 and which is incorporated herein in its entiretyby reference.

FIELD

The present invention relates to an inspection apparatus and a methodfor performing a measurement. In particular, it relates to an inspectionapparatus and method for inspecting reticle substrates, reticle blanksand/or patterned reticles.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Multiple layers, each having a particular pattern and materialcomposition, are applied to define functional devices andinterconnections of the finished product.

In lithographic processes, it is desirable frequently to makemeasurements of the reticle at different stages of its manufacture,e.g., to monitor reticle quality. In particular, it is desirable tomonitor for defects on the reticle. This presents particulardifficulties for EUV reticles tuned for patterning and reflectingradiation in the EUV regime (e.g., having a wavelength in the region ofabout 13.5 nm). Such defects may take many forms, including pitting ofthe reticle surface, a contaminant (particle) on the reticle or anundesired variation of the pitch of the reticle multilayer. Any suchreticle defects may be imaged onto the product (wafer) rendering thefinal device defective. Various tools for making such measurements areknown, including scanning electron microscopes and various forms ofscatterometers.

The known scatterometers tend to use light in the visible or near-IRwave range, which limits the resolution of the measurements andtherefore the size of any defects which can be detected with anyaccuracy. On the other hand, the dimensions of these defects are sosmall that they cannot be imaged by optical metrology techniques. Whilescanning electron microscopy (SEM) is able to resolve these defectsstructures directly, SEM is much more time consuming than opticalmeasurements.

It is known to decrease the wavelength of the radiation used duringmetrology, i.e., by moving towards extreme ultraviolet (EUV) having awavelength in the region of radiation of 13.5 nm. Such radiation canbetter resolve small defects. Also, such radiation is of a similar orsame wavelength as the imaging radiation used in EUV lithography andtherefore the ability to detect defects with such radiation isindicative that the defect will be imaged. Furthermore, it isadvantageous to inspect a reticle blank with 13.5 nm radiation becausethe multilayer mirror of which it is comprised is designed to reflect13.5 nm, which means that defects in the multilayer structure can bedetected more easily.

However, it is known that inspection using inspection radiation having awavelength of 13.5 nm suffers from a low number of photons impinging onthe sensor and, thus, relatively long measurement times.

It would be desirable to improve measurement speed of (e.g., EUV)reticle inspection, including inspection of the bare substrate, thereticle blank (with multilayer applied) or of the patterned reticle.

SUMMARY

According to a first aspect of the present invention, there is method ofinspection for detecting defects on a substrate for a reflectivereticle, comprising: performing said inspection using first inspectionradiation obtained from a high harmonic generation source and having oneor more first wavelengths within a first wavelength range of between 20nm and 150 nm.

According to a second aspect of the present invention, there is providedmethod of inspection for detecting defects on a substrate for areflective reticle, comprising: performing a coarse inspection usingfirst inspection radiation having one or more first wavelengths within afirst wavelength range; and performing a fine inspection using secondinspection radiation having one or more second wavelengths within asecond wavelength range, said second wavelength range comprisingwavelengths shorter than said first wavelength range.

The claims dependent from claim 1, relating to the first aspect, whichdefine wavelength ranges, angles of incidence, and number of wavelengthsfor the inspection radiation are all applicable to the first inspectionof this second aspect.

According to a third aspect of the invention, there is provided aninspection apparatus operable to perform the method of the first orsecond aspects.

Further aspects, features and advantages of the invention, as well asthe structure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus;

FIG. 2 depicts a metrology apparatus using a HHG source adaptableaccording to an embodiment of the invention;

FIG. 3 is a plot showing a typical HHG emission spectrum for an HHGsource using neon gas;

FIG. 4 comprises graphs of reflectivity R (%) on the y axis againstwavelength λ (nm) on the x axis for angles of incidence with respect tonormal of (a) 0 degrees, (b) 45 degrees and (c) 60 degrees.

FIG. 5 is a flowchart of a method according to a first embodiment of theinvention; and

FIG. 6 is a flowchart of a method according to a second embodiment ofthe invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV or EUV radiation), apatterning device support or support structure (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; two substratetables (e.g., a wafer table) WTa and WTb each constructed to hold asubstrate (e.g., a resist coated wafer) W and each connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., including one or more dies) of the substrate W. Areference frame RF connects the various components, and serves as areference for setting and measuring positions of the patterning deviceand substrate and of features on them.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support MT may be a frame or a table, for example,which may be fixed or movable as required. The patterning device supportmay ensure that the patterning device is at a desired position, forexample with respect to the projection system.

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive patterning device). Alternatively, theapparatus may be of a reflective type (e.g., employing a programmablemirror array of a type as referred to above, or employing a reflectivemask). Examples of patterning devices include masks, programmable mirrorarrays, and programmable LCD panels. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.” The term “patterning device” can also beinterpreted as referring to a device storing in digital form patterninformation for use in controlling such a programmable patterningdevice.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

In operation, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may for example include an adjuster AD for adjustingthe angular intensity distribution of the radiation beam, an integratorIN and a condenser CO. The illuminator may be used to condition theradiation beam, to have a desired uniformity and intensity distributionin its cross section.

The radiation beam B is incident on the patterning device MA, which isheld on the patterning device support MT, and is patterned by thepatterning device. Having traversed the patterning device (e.g., mask)MA, the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the second positioner PW and position sensor IF (e.g., aninterferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WTa or WTb can be moved accurately, e.g.,so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor (which is not explicitly depicted in FIG. 1) can be usedto accurately position the patterning device (e.g., mask) MA withrespect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment mark may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus could be used in a variety of modes. In a scanmode, the patterning device support (e.g., mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The speed and direction of the substrate table WTrelative to the patterning device support (e.g., mask table) MT may bedetermined by the (de-)magnification and image reversal characteristicsof the projection system PS. In scan mode, the maximum size of theexposure field limits the width (in the non-scanning direction) of thetarget portion in a single dynamic exposure, whereas the length of thescanning motion determines the height (in the scanning direction) of thetarget portion. Other types of lithographic apparatus and modes ofoperation are possible, as is well-known in the art. For example, a stepmode is known. In so-called “maskless” lithography, a programmablepatterning device is held stationary but with a changing pattern, andthe substrate table WT is moved or scanned.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo substrate tables WTa, WTb and two stations—an exposure station EXPand a measurement station MEA—between which the substrate tables can beexchanged. While one substrate on one substrate table is being exposedat the exposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory stepscarried out. This enables a substantial increase in the throughput ofthe apparatus. The preparatory steps may include mapping the surfaceheight contours of the substrate using a level sensor LS and measuringthe position of alignment markers on the substrate using an alignmentsensor AS. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations, relative to reference frame RF. Other arrangements areknown and usable instead of the dual-stage arrangement shown. Forexample, other lithographic apparatuses are known in which a substratetable and a measurement table are provided. These are docked togetherwhen performing preparatory measurements, and then undocked while thesubstrate table undergoes exposure.

Metrology tools may be developed which use sources that emit radiationin “soft X-ray” or EUV range, for example having wavelengths between 1nm and 50 nm. Examples of such sources include Discharge Produced Plasmasources, Laser Produced Plasma Sources or High-order Harmonic Generation(HHG) sources. HHG sources are known to be able to provide large flux ofcollimated photons (high luminance) in the emitted light.

HHG sources used in metrology applications are illustrated and furtherdescribed in United States Patent Application 20170315456, which ishereby incorporated in their entirety by reference. In metrologyapplications, such HHG sources may be used (for example) in normalincidence, very close to normal incidence (e.g., within 10 degrees fromnormal), at a grazing incidence (e.g., within 20 degrees from surface),at an arbitrary angle or at multiple angles (to obtain more measurementinformation in a single capture).

FIG. 2 illustrates an metrology arrangement showing the radiation source430 in more detail. Radiation source 430 is an HHG source for generatingEUV radiation based on high harmonic generation (HHG) techniques. Maincomponents of the radiation source 430 are a pump laser 431 and an HHGmedium, such as a HHG gas cell 432 (a HHG solid surface medium may alsobe used). A gas supply 434 supplies suitable gas to the gas cell, whereit is optionally ionized by an electric source (not shown). The pumplaser 431 may be for example a fiber-based laser with an opticalamplifier, producing pulses of infrared radiation lasting less than 1 ns(1 nanosecond) per pulse, with a pulse repetition rate up to severalmegahertz, as required. The wavelength may be for example in the regionof 1 μm (1 micron). The laser pulses are delivered as a pump radiationbeam 440 to the HHG gas cell 432, where a portion of the radiation isconverted to higher frequencies. From the HHG gas cell 432 emerges abeam of measurement radiation 442 including coherent radiation of thedesired wavelength or wavelengths.

The measurement radiation 442 may contain multiple wavelengths. If theradiation is also monochromatic, then measurement calculations(reconstruction) may be simplified, but it is easier with HHG to produceradiation with several wavelengths. These are matters of design choice,and may even be selectable options within the same apparatus. Differentwavelengths will, for example, provide different levels of contrast whenimaging structure of different materials. For inspection of metalstructures or silicon structures, for example, different wavelengths maybe selected to those used for imaging features of (carbon-based) resist,or for detecting contamination of such different materials.

One or more filtering devices 444 may be provided. For example a filtersuch as a thin membrane of Aluminum (Al) may serve to cut thefundamental IR radiation (and undesired long-wavelength harmonics) frompassing further into the inspection apparatus. A grating may be providedto select one or more specific harmonic wavelengths from among thosegenerated in the gas cell 432. Some or all of the beam path may becontained within a vacuum environment, bearing in mind that EUVradiation is absorbed when traveling in air. The various components ofradiation source 430 and illumination optics can be adjustable toimplement different metrology ‘recipes’ within the same apparatus. Forexample different wavelengths and/or polarization can be madeselectable.

From the radiation source 430, the filtered beam enters an inspectionchamber where the substrate 462 is held for inspection by substratesupport 414. The atmosphere within inspection chamber is maintained nearvacuum by vacuum pump 452, so that the soft X-ray radiation can passwithout undue attenuation through the atmosphere. The illuminationsystem includes one or more optical elements 454 for focusing theradiation into a focused beam 456, and may comprise for example atwo-dimensionally curved mirror, or a series of one-dimensionally curvedmirrors, as described in the prior applications mentioned above.Diffraction gratings such as the spectroscopic gratings can be combinedwith such mirrors, if desired. The focusing is performed to achieve around or elliptical spot under 10 μm in diameter, when projected ontothe structure of interest. Substrate support 414 comprises for examplean X-Y translation stage and a rotation stage, by which any part of thesubstrate W can be brought to the focal point of beam to in a desiredorientation. Thus the radiation spot S is formed on the structure ofinterest. The radiation scattered 408 from the structure of interest isthen detected by detector 460. This detector may be large enough tocapture higher diffraction orders in addition to the specularlyreflected radiation (zeroth order). Alternatively, one or more separatedetectors are used for the higher diffraction orders (in combinationwith or instead of detector 460). The detector configuration willlargely depend on the detection and metrology methods actually used, andwhether the substrate being measured has been processed or not, forexample.

Such an arrangement may be used to inspect any type of substrate. In theexamples described herein, the substrate being inspected comprise areticle (or mask) substrate, and more specifically reflective reticlessuch as those typically used for EUV lithography. EUV lithography usesimaging radiation having a wavelength smaller than 20 nm, and morespecifically about 13.5 nm, thereby enabling printing of smallerfeatures compared to using larger wavelength radiation. However,radiation at these wavelengths will be absorbed by transmissive opticsand therefore reflective optics are used throughout the system includingthe reticle. Each reticle should be inspected to identify and/orcharacterize defects, as such defects may be imaged onto a wafer whenused, resulting in defective devices (loss of yield). Reticle substratesmay be inspected at different stages of manufacture; for example, theinspection stages may include:

-   -   substrate inspection, i.e., inspection of an unprocessed        substrate,    -   multilayer inspection, i.e., inspection of a “blank” comprising        a substrate with at least the multilayer (reflective coating)        applied, typically also having a back side coating and        unpatterned absorber layer applied, and    -   defect on absorber inspection, i.e., inspection of a patterned        substrate on which the absorber layer has been patterned.        The concepts described herein are equally applicable to each of        these inspection stages.

A single defect on a reticle has the potential to destroy the completeintegrated circuit printed on the wafer, and hence a defect free reticleis important for enabling high volume manufacturing (HVM) in EUVmetrology. The defects may comprise amplitude defects such as surfacebumps, particles or pits which generate contrast changes at the waferlevel during exposure, and phase defects such as bumps or pits buriedwithin or below the multilayer, which cause a phase change in thereflected radiation.

There have been several methods used or described for EUV reticle defectmetrology, but none are sufficient for HVM using EUV tools. Typicalexamples use optical scatterometry, scanning electron microscopy (SEM)or atomic force microscopy (AFM). SEM is capable of measuring thedefects with very high resolution in a plane, while AFM is very accuratein measuring height related defects. However, each of these methods aretoo slow for HVM. Therefore optical microscopy/scatterometry is oftenthe preferred method for defect detection in reticle metrology, due toits relative faster speed. However, this higher speed is only reallyobtainable when using DUV and higher wavelengths, which are resolutionlimited (i.e., unable to detect or characterize defects at the requiredresolution). Inspection at EUV and shorter wavelengths are photonlimited (i.e., the amount of light available at these wavelengths islimited) which limits the throughput time, and is thus not economicallyviable for HVM.

Additionally, actinic radiation may be preferred as some printabledefects (i.e., defects on the reticle which are imaged by EUV radiationas defects on the wafer) will not always be detected by radiation atother wavelengths, or using other metrology techniques. Actinicinspection radiation, in this context, is inspection radiation forreticle inspection which essentially has the same characteristics(particularly wavelength) as the imaging radiation for which the reticleis designed to pattern. By way of specific example, the actinicwavelength may be in the region of 13.5 nm for EUV reticles. However,the reflectivity of the EUV multilayer stacks applied to each EUVreticle substrate is typically optimized for reflection of 13.5 nmradiation at a 6 degree angle of incidence from surface normal. Fordefect detection with high throughput at a 13.5 nm wavelength, a veryhigh brightness source is required which, should it even exist, is noteconomically viable for inspection applications. More specifically, whenusing a high harmonic generation (HHG) radiation source, only a verysmall number of harmonics, often only one, correlates with the 13.5 nmradiation. Because of this, the amount of radiation that is actuallyavailable at 13.5 nm is small and measurement will be slow. As such,there is no technique at present which can detect reticle defects withthe required resolution and with the desired throughput.

FIG. 3 shows a typical HHG emission spectrum for an HHG source usingneon gas as a HHG generation medium, illustrating the above issue. Thefigure comprises intensity I on the y-axis against wavelength λ (nm) onthe x-axis. It can be seen from the figure that only a small number ofpeaks correlate with the desired 13.5 nm wavelength; the remaining peaksare typically filtered out.

To increase the number of photons available for measurement, it isproposed to perform defect inspection on a substrate using inspectionradiation at one or more inspection wavelengths in a radiation rangebetween 20 nm and 150 nm; more specifically the radiation range may bebetween 20 nm and 90 nm, between 20 nm and 70 nm, between 20 nm and 50nm or between 20 nm and 35 nm.

The inspection radiation may comprise a wavelength that is approximatelyan integer multiple of an actinic wavelength for which the reticle isdesigned; in particular where the integer is 2 or 3. “Approximately”with respect to any stated wavelength value may define a range of +/−5nm or +/−2 nm centered on the stated wavelength value. As such, theinspection radiation may comprise a plurality of wavelengths within awavelength range as defined above, and which includes one wavelengthwhich is approximately an integer multiple of an actinic wavelength. Anadvantage of this is that by including radiation at, for example, 27 nm(and, for example, at an “actinic” angle of incidence, which may be 6degrees with respect to normal) approximately half of the layers of thesubstrate multilayer will be used for reflection and, therefore defectsin the multilayer structure may be detectable.

To further increase the number of photons available, it is proposed, inan embodiment, that the inspection radiation specifically comprisesradiation at a plurality of wavelengths. The brightness (amount ofradiation available) at lower harmonics (higher wavelengths) can beorders of magnitude higher compared to that of the higher harmonics(lower wavelengths). Additionally, the emission spectrum can also beoptimized for maximum brightness at desired wavelengths.

More specifically, the reflection spectrum of the substrate beinginspected can be expected to be relatively constant over a few nm ofwavelength range, and therefore many harmonics of a HHG source can beused for the inspection. Additionally, one or more HHG generationparameters may be optimized for brightness (i.e., amount of radiationproduced) at the wavelength range of interest. For example, the pressureand/or species of the HHG generating medium (e.g., HHG generating gascomprised within HHG gas cell 432 in FIG. 2) and/or the wavelength ofthe pump laser used to excite the HHG generation medium may be sooptimized. As such, the HHG generation parameters may be optimized forbrightness in the 20-40 nm wavelength range, rather than in the rangeillustrated in FIG. 3. The publication “High flux coherentsuper-continuum soft X-ray source driven by a single-stage, 10 mJ,Ti:sapphire amplifier-pumped OPA” Ding et al, OPTICS EXPRESS, Vol. 22,No. 5 shows different HHG emission spectra for different gas species(argon, neon, helium) at varying pressure and driving wavelength(wavelength of the pump laser), which illustrates how one or more HHGgeneration parameters, such as gas species, may be optimized for apreferred wavelength range. The content of this publication isincorporated herein by reference in its entirety.

Another variable for increasing the number of photons available formeasurement is the angle of incidence of the measurement radiation withrespect to the substrate surface. FIG. 4, which shows graphs ofreflectivity R (% on a log scale) on the y axis against wavelength λ(nm) on the x axis for angles of incidence, with respect to normal, of:(a) 0 degrees, (b) 45 degrees and (c) 60 degrees. It can be observedthat there is approximately 1% reflection from the multilayer atwavelengths in the range 25 nm-30 nm, at about normal incidence. Byincreasing the angle of incidence from normal to 45 degrees or 60degrees with respect to normal, reflectivity at higher wavelengths(e.g., within the aforementioned 25 nm-30 nm range) can be increased byan order of magnitude; i.e., FIG. 4(c) shows reflectivity in the regionof about 10% within this wavelength range.

As such, in an embodiment it is proposed that the inspection beperformed with the inspection radiation impinging the substrate surfaceat an angle (relative to surface normal) of greater than 10 degrees,greater than 20 degrees, greater than 30, greater than 40 degrees orgreater than 50 degrees; e.g., in a range of between 40 and 70 degrees.

In another embodiment, it is proposed to perform defect inspection on areticle substrate in two stages: a coarse inspection using firstinspection radiation having one or more first wavelengths that are allwithin a first wavelength range, e.g., between 20 nm and 150 nm; and afine inspection using second inspection radiation having one or moresecond wavelengths that are all within a second wavelength range, e.g.,less than 20 nm.

The first inspection radiation may comprise any of the parameter valuesand concepts relating to the inspection radiation and inspectiontechniques described in the aforementioned embodiments. As such, thefirst inspection radiation may comprise any of the narrower ranges,and/or may include a wavelength which is an integer multiple of anactinic wavelength, in the same manner as described in relation to theabove embodiment. The first inspection radiation may comprise multiplewavelengths. Additionally, the coarse inspection may be performed at anangle (relative to surface normal) of greater than 10 degrees, greaterthan 20 degrees, greater than 30, greater than 40 degrees or greaterthan 50 degrees; e.g., the region of between 40 and 60 degrees, asalready described.

The second inspection radiation may comprise actinic radiation, suchthat said second wavelength is an actinic wavelength. In a specificembodiment, the second inspection radiation may have a wavelength ofapproximately 13.5 nm

The coarse inspection may be performed for fast scatterometry/imagingover the substrate. A fine inspection may then be performed, ifrequired, using an actinic wavelength (e.g., 13.5 nm) for a more precisecharacterization of any defect detected during the coarse inspectionand/or to ascertain whether the defect will be imaged in a productionprocess. Even smaller wavelengths (e.g., smaller than 13.5 nm) may beused to obtain even greater resolution during the fine inspection.

In such an embodiment, the HHG generating medium may be individuallyoptimized for brightness at each of the wavelength ranges of interestcorresponding to both the coarse inspection and fine inspection. Forexample, a first HHG generating medium may be used for the coarseinspection stage and a second HHG generating medium (different to thefirst HHG generating medium) used for the fine inspection stage. In aspecific example, the first HHG generating medium may be optimized forbrightness in the 20-40 nm wavelength range, and the second HHGgenerating medium optimized for brightness at the actinic wavelength(e.g., approximately 13.5 nm) or other wavelength used during fineinspection.

In a further embodiment, instead of using a HHG source, an inverseCompton scattering (ICS) source may be used to generate the inspectionradiation for at least the coarse inspection stage (and possibly forboth coarse and fine inspection stages). Such an ICS source isdescribed, for example, in WO 2017/025392 which is hereby incorporatedby reference. For more detail of the implementation of an ICS source,reference is made to W S Graves et al, “Compact x-ray source based onburst-mode inverse Compton scattering at 100 kHz”, Physical ReviewSpecial Topics—Accelerators and Beams 17, 120701 (2014). The contents ofthe Graves et al reference are incorporated herein by reference.

The concept of coarse inspection using inspection radiation havingrelatively longer wavelengths than those used for fine inspection may beimplemented in different ways. In one embodiment, the coarse inspectionand fine inspection may be performed sequentially per substrate. FIG. 5is a flowchart illustrating such an embodiment. At step 300, a newsubstrate is loaded for inspection and at step 310, a coarse inspectionis performed on a substrate to identify regions of interest havingpotential defects (i.e., any regions on the substrate for which thecoarse inspection highlights a possible defect). At step 320, a fineinspection is performed on only those regions of interest identified atstep 310. At step 325, the defect is characterized, e.g., by performinga reconstruction based on an image (e.g., a diffraction image) obtainedin the fine inspection stage, so as to determine the type of defectand/or quantify its effect on any IC manufacturing process using thereticle. Should no potential defects be identified in the coarseinspection, then a fine inspection need not be performed on thatsubstrate. The method returns to step 300 for the next substrate to bemeasured.

In an alternative embodiment, the inspection method may comprise acalibration step for which a one-to-one mapping of measurementsignatures (e.g., diffraction image or pupil image or any other imagedsignature indicative of a defect) using the coarse inspection radiationand the fine inspection radiation is made, thereby generating a libraryof corresponding defect signatures at the two wavelengths. The librarycan then be accessed during the manufacturing (HVM) stage instead ofperforming a fine inspection. FIG. 6 is a flowchart illustrating such anembodiment. In a calibration stage 330, at step 340, a number ofdifferent types of defect are inspected using relatively longerwavelength radiation (i.e., in the first wavelength range) and aresultant first diffraction image or other defect signature is recorded.At step 350, each of these defects are then inspected using relativelyshorter wavelength radiation (i.e., in the second wavelength range) and,for each defect, the resultant second diffraction image or other defectsignature is mapped to the corresponding first diffraction image orother defect signature for that defect. Then, during a manufacturingstage 360, a substrate is loaded (step 370) and a coarse inspection(step 380) is performed using radiation in the first wavelength range.At step 390, the library is accessed to identify the closest firstsignature match to the defect signature obtained at step 380. Thecorresponding second defect signature to this closest first signature isthen used, at step 395, to characterize the defect (e.g., by way ofreconstruction), and the method returns to step 370 for the nextsubstrate.

The inspection methods used herein, including at each of the coarseinspection stage and fine inspection stage, may comprise any suitablemethod for measuring defects on a substrate. Such methods may comprise,for example, reflecting the inspection radiation off the substratesurface and imaging the reflected radiation. This reflected radiationcan be compared to a reference level obtained from inspecting a defectfree substrate or area thereof. Any substantial deviation from thereference can be taken to be indicative of a defect. As an alternative,the dark field (e.g., an imaged region where there is no scatteredradiation in the absence of a defect) may be monitored, and anyradiation detected in the dark field may be considered indicative of adefect. Such methods and other suitable methods can be found, forexample, in Nakasuji et al, Development of Coherent Extreme-UltravioletScatterometry Microscope with High-Order Harmonic Generation Source forExtreme-Ultraviolet Mask Inspection and Metrology, Japanese Journal ofApplied Physics 51 (2012) and Na et al, Application of actinic maskreview system for the preparation of HVM EUV lithography with defectfree mask, Proc. of SPIE Vol. 10145. Both of these publications arehereby incorporated by reference in their entirety.

In Na et al, a scanning based imaging system is used which providesaerial images. Firstly, a HHG source generates 13.5 nm EUV light, whichis delivered to the reticle (a multilayer mirror) and focused thereon bya zoneplate. Secondly, while the substrate stage scans the mask in X andY directions, while each point intensity is detected at the detector andconverted to electrical signals. Thirdly, the signals are digitized atthe analog-to-digital converter and sent to the imaging processing unit.Finally, aerial images are reconstructed during the mask scans, thisprocess being repeated until the scan ends. Inspection of the aerialimage should reveal defects.

In Nakasuji et al, a coherent EUV scatterometry microscope (CSM), isused, which comprises a lensless system for actinic inspection andmetrology. At the CSM, the mask is exposed to a coherent EUV light. Acharge-coupled-device (CCD) camera records diffraction and scatteringintensities from the mask directly, which contains amplitude informationin the frequency space. CD values are evaluated using the diffractionintensities from the periodic patterns, and aerial images arereconstructed by the iterative calculation of Fourier andinverse-Fourier transforms. Defects are also observed from thediffraction images, e.g., as intensity spikes between the diffractionorders. Methods include reconstructing the aperiodic pattern image,reconstructing the phase structure as the amplitude and the phase image,and reconstructing a defect image.

Further embodiments are provided in the subsequent numbered clauses:

1. A method of inspection for detecting defects on a substrate for areflective reticle, comprising:

-   -   performing said inspection using first inspection radiation        obtained from a high harmonic generation source and having one        or more first wavelengths within a first wavelength range of        between 20 nm and 150 nm.

2. A method as defined in clause 1, wherein said inspection stepcomprises using said first inspection radiation at an angle of incidenceonto said substrate greater than 10 degrees with respect to the surfacenormal.

3. A method as defined in clause 1, wherein said inspection stepcomprises using said first inspection radiation at an angle of incidenceonto said substrate greater than 40 degrees with respect to the surfacenormal.

4. A method as defined in any preceding clause, wherein said one or morefirst wavelengths comprise a plurality of wavelengths.

5. A method as defined in any preceding clause, wherein the one or morefirst wavelengths comprise a wavelength which is approximately aninteger multiple of the wavelength of an actinic wavelength for whichthe reticle substrate has been optimized.

6. A method as defined in clause 5, wherein said integer is 2 or 3.

7. A method as defined in any preceding clause, wherein said firstwavelength range is between 20 nm and 40 nm.

8. A method as defined in any preceding clause, wherein one or more highharmonic generation parameters of the high harmonic generation source isoptimized for maximal photon generation at said one or more firstwavelengths; the one or more high harmonic generation parameterscomprising one or more of:

-   -   the species of a high harmonic generation generating medium        comprised within the high harmonic generation source for        generating the first inspection radiation;    -   the pressure of the high harmonic generation generating medium;        and/or    -   the wavelength of a pump laser used to excite the high harmonic        generation generating medium.

9. A method as defined in any preceding clause, wherein the reflectivereticle substrate comprises one of a:

-   -   a bare substrate,    -   a reticle blank with multilayer applied, or    -   a patterned substrate comprising an absorber layer into which a        pattern has been applied.

10. A method as defined in any preceding clause, wherein said performingan inspection using first inspection radiation is performed as part of acoarse inspection; and said method further comprises:

-   -   performing a fine inspection using second inspection radiation        having one or more second wavelengths within a second wavelength        range, said second wavelength range comprising wavelengths        shorter than said first wavelength range.

11. A method as defined in clause 10, wherein said one or more firstwavelengths are all comprised within the first wavelength range and saidone or more second wavelengths are all comprised within the secondwavelength range.

12. A method as defined in clause 10 or 11, wherein said firstwavelength range and said second wavelength range are non-overlapping.

13. A method as defined in any of clauses 10 to 12, wherein the secondinspection radiation comprises actinic radiation, such that said secondwavelength comprises the wavelength for which the reflective reticle hasbeen optimized.

14. A method as defined in any of clauses 10 to 13, wherein said secondwavelength range comprises wavelengths smaller than 20 nm.

15. A method as defined in any of clauses 10 to 14, wherein said secondinspection radiation comprises a second wavelength of approximately 13.5nm.

16. A method as defined in any of clauses 10 to 15, wherein the coarseinspection is performed over the substrate surface to identify regionsof interest indicative of the presence of a defect; and the fineinspection is performed only for the regions of interest forcharacterization of a defect.

17. A method as defined in any of clauses 10 to 16, wherein the coarseinspection and fine inspection are performed sequentially on eachsubstrate being inspected.

18. A method as defined in any of clauses 10 to 16, wherein the coarseinspection and fine inspection are performed in a calibration step oncalibration substrates to form a library of corresponding first defectsignatures obtained from said coarse inspection and second defectsignatures obtained from said fine inspection, said library being usedto find corresponding second defect signatures and thereforecharacterize a defect based on a first defect signature obtained in acoarse inspection during an inspection step.

19. A method as defined in any of clauses 10 to 18, wherein one or morehigh harmonic generation parameters of the high harmonic generationsource is optimized for maximal photon generation in said firstwavelength range during said coarse inspection and in said secondwavelength range during said fine inspection; the one or more highharmonic generation parameters comprising one or more of:

-   -   the species of a high harmonic generation generating medium        comprised within the high harmonic generation source for        generating the first inspection radiation;    -   the pressure of the high harmonic generation generating medium;        and/or    -   the wavelength of a pump laser used to excite the high harmonic        generation generating medium.

20. A method of inspection for detecting defects on a substrate for areflective reticle, comprising:

-   -   performing a coarse inspection using first inspection radiation        having one or more first wavelengths within a first wavelength        range; and    -   performing a fine inspection using second inspection radiation        having one or more second wavelengths within a second wavelength        range, said second wavelength range comprising wavelengths        shorter than said first wavelength range.

21. A method as defined in clause 20, wherein the coarse inspection isperformed over the substrate surface to identify regions of interestindicative of the presence of a defect; and the fine inspection isperformed only for the regions of interest for characterization of adefect.

22. A method as defined in clause 20 or 21, wherein the coarseinspection and fine inspection are performed sequentially on eachsubstrate being inspected.

23. A method as defined in clause 20 or 21, wherein the coarseinspection and fine inspection are performed in a calibration step oncalibration substrates to form a library of corresponding first defectsignatures obtained from said coarse inspection and second defectsignatures obtained from said fine inspection, said library being usedto find corresponding second defect signatures and thereforecharacterize a defect based on a first defect signature obtained in acoarse inspection during an inspection step.

24. A method as defined in any of clauses 20 to 23, wherein said firstinspection radiation and said second inspection radiation are obtainedfrom a high harmonic generation source.

25. A method as defined in any of clauses 20 to 23, wherein said firstinspection radiation and said second inspection radiation are obtainedfrom an inverse Compton scattering source.

26. An inspection apparatus operable to perform the method of any ofclauses 20 to 23.

27. An inspection apparatus as defined in clause 26, comprising a highharmonic generation source for generation of said first inspectionradiation and said second inspection radiation.

28. An inspection apparatus as defined in clause 27, comprising aplurality of high harmonic generation media, comprising a first highharmonic generation medium for generation of said first inspectionradiation in said first wavelength range during said coarse inspectionand a second high harmonic generation medium for generation of saidsecond inspection radiation in said second wavelength range during saidfine inspection.

29. An inspection apparatus as defined in clause 28, operable such thatthe pressure of the first high harmonic generation medium and or thewavelength of the pump laser used to excite the first high harmonicgeneration medium is further optimized for generation of said firstinspection radiation in said first wavelength range during said coarseinspection; and/or the pressure of the second high harmonic generationmedium and or the wavelength of the pump laser used to excite the secondhigh harmonic generation medium is further optimized for generation ofsaid second inspection radiation in said second wavelength range duringsaid fine inspection.

30. An inspection apparatus as defined in clause 27, comprising aninverse Compton scattering source for generation of said firstinspection radiation and said second inspection radiation.

31. An inspection apparatus operable to perform the method of any ofclauses 1 to 19.

32. An inspection apparatus as defined in any of clauses 26 to 31,comprising:

-   -   a substrate holder for holding the substrate,    -   projection optics for projecting the inspection radiation onto        the substrate; and    -   a sensor for sensing the inspection radiation having been        scattered and/or reflected from the substrate.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used in relation to the lithographicapparatus, unless otherwise specified, encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultraviolet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A method of inspection for detecting defects on a substrate for areflective reticle, the method comprising performing the inspectionusing inspection radiation obtained from a high harmonic generationsource and having one or more wavelengths within a wavelength range ofbetween 20 nm and 150 nm.
 2. The method as claimed in claim 1, whereinthe performing the inspection comprises using the inspection radiationat an angle of incidence onto the substrate greater than 10 degrees withrespect to a normal of a surface of the substrate.
 3. The method asclaimed in claim 1, wherein the one or more wavelengths comprise aplurality of wavelengths.
 4. The method as claimed in claim 1, whereinthe one or more wavelengths comprise a wavelength which is approximatelyan integer multiple of the wavelength of an actinic wavelength for whichthe reticle substrate has been optimized.
 5. The method as claimed inclaim 1, wherein the wavelength range is between 20 nm and 40 nm.
 6. Themethod as claimed in claim 1, wherein one or more high harmonicgeneration parameters of the high harmonic generation source isoptimized for maximal photon generation at the one or more wavelengths,the one or more high harmonic generation parameters comprising: thespecies of a high harmonic generation generating medium comprised withinthe high harmonic generation source for generating the inspectionradiation; and/or the pressure of the high harmonic generationgenerating medium; and/or the wavelength of a pump laser used to excitethe high harmonic generation generating medium.
 7. The method as claimedin claim 1, wherein the reflective reticle substrate comprises one of: abare substrate, a reticle blank with a multilayer applied, or apatterned substrate comprising an absorber layer into which a patternhas been applied.
 8. The method as claimed in claim 1, wherein theperforming an inspection using inspection radiation is performed as partof a coarse inspection; and the method further comprises: performing afine inspection using inspection radiation having one or morewavelengths within a fine inspection wavelength range comprisingwavelengths shorter than the wavelength range of between 20 nm and 150nm.
 9. The method as claimed in claim 8, wherein the one or morewavelengths of the coarse inspection are all comprised within thewavelength range of between 20 nm and 150 nm and the one or morewavelengths of the fine inspection are all comprised within the fineinspection wavelength range.
 10. The method as claimed in claim 8,wherein the wavelength range of between 20 nm and 150 nm and the fineinspection wavelength range are non-overlapping.
 11. The method asclaimed in claim 8, wherein the inspection radiation of the fineinspection comprises actinic radiation, such that the one or morewavelengths of the fine inspection comprises the wavelength for whichthe reflective reticle has been optimized.
 12. The method as claimed inclaim 8, wherein the coarse inspection is performed over the a surfaceof the substrate to identify regions of interest indicative of thepresence of a defect; and the fine inspection is performed only for theregions of interest for characterization of a defect.
 13. A method ofinspection for detecting defects on a substrate for a reflectivereticle, the method comprising: performing a coarse inspection usingfirst inspection radiation having one or more first wavelengths within afirst wavelength range; and performing a fine inspection using secondinspection radiation having one or more second wavelengths within asecond wavelength range, the second wavelength range comprisingwavelengths shorter than the first wavelength range.
 14. An inspectionapparatus, comprising: a high harmonic generation source configured tooutput inspection radiation having one or more wavelengths within awavelength range of between 20 nm and 150 nm; an optical elementconfigured to receive the inspection radiation and provide theinspection radiation to a surface of a substrate for a reflectivereticle; and a detector configured to detect radiation from thesubstrate for detecting defects on the substrate.
 15. The apparatus ofclaim 14, wherein the optical element is arranged so that the inspectionradiation is arranged to be at an angle of incidence onto the substrategreater than 10 degrees with respect to the surface normal.
 16. Theapparatus of claim 14, wherein the wavelength range is between 20 nm and40 nm.
 17. The apparatus of claim 14, wherein the one or morewavelengths comprise a wavelength which is approximately an integermultiple of the wavelength of an actinic wavelength for which thereticle substrate has been optimized.
 18. The apparatus of claim 14,configured to perform an inspection using the inspection radiation aspart of a coarse inspection, and to perform a fine inspection usinginspection radiation having one or more wavelengths within a fineinspection wavelength range comprising wavelengths shorter than thewavelength range of between 20 nm and 150 nm.
 19. The apparatus of claim14, wherein the one or more wavelengths of the coarse inspection are allcomprised within the wavelength range of between 20 nm and 150 nm andthe one or more wavelengths of the fine inspection are all comprisedwithin the fine inspection wavelength range.
 20. The apparatus of claim14, wherein the wavelength range of between 20 nm and 150 nm and thefine inspection wavelength range are non-overlapping.